The present invention relates to instruments that measure the concentration of fine particles in gas, known as condensation nuclei counters or condensation particle counters. More particularly, the invention concerns a condensation particle counter using a water-immiscible working fluid, and a means for removing water from the working fluid.
For years, condensation particle counters have been used successfully in aerosol research More recently they have gained acceptance in industrial settings that require detecting fine and ultrafine airborne particles. In "clean rooms" used for the production of semi-conductor chips, disk drives, optical systems and pharmaceuticals, there is an increasing need to detect and measure concentrations of submicrometer particles.
Condensation particle counters operate according to the principle that supersaturated vapor condenses on fine particles to form aerosol droplets Typically, room air (or other gas being monitored) is drawn through a horizontal chamber inside a saturator block. A working fluid, usually n-butyl alcohol (butanol), evaporates into the gas stream, saturating the stream with alcohol vapor. From the saturator, the gas is drawn into a vertical condenser tube and cooled sufficiently to supersaturate the vapor. Vapor condenses onto the particles, forming aerosol droplets much larger than the particles. From the condenser, the gas stream passes an optical detector that senses the aerosol droplets traveling through a viewing volume defined by a laser and associated optics. For further information on this type of device, reference is made to U.S. Pat. No. 4,790,650 (Keady), assigned to the assignee of this application.
When a continuous flow condensation particle counter is used to monitor room air, the air stream incorporates water vapor as well as working fluid vapor, and the condenser forms water as well as working fluid condensate. Over extended operation of the counter, increasing amounts of water are absorbed by the butanol. As the water concentration increases, instrument performance is degraded. Accordingly, it is necessary to periodically drain the butanol and water mixture from the device, and add pure butanol. The required frequency of working fluid replacement depends on the relative humidity of the air, but relative humidity levels of thirty percent or more can give rise to substantial fluid degradation. Further problems arise from the chemically reactive nature, flammability and disagreeable odor of butanol.
Therefore, it is an object of the present invention to provide a continuous flow condensation particle counter incorporating means for substantially preventing the introduction of water into the working fluid as the working fluid is recirculated through the particle counter.
Another object is to provide, in a condensation particle counter, a non-toxic, non-flammable and chemically inert working fluid in which water is immiscible.
A further object of the invention is to provide a simple and reliable system for collecting and removing water formed in a condensation particle counter.
Yet another object is to provide a continuous flow condensation particle counter particularly well suited for long term, low maintenance operation in high humidity environments.